1. Field of the Invention
This invention relates generally to optical telecommunication systems and, more particularly, to receivers employed in such systems.
2. Description of the Related Art
Polarization diverse optical circuits use a polarization beam splitter (PBS) to split an incoming optical signal into two orthogonal and spatially separated polarization states. These output polarization states are often the transverse electric (TE) and transverse magnetic (TM) polarization states of a planar optical waveguide circuit.
A PBS in planar lightwave circuits (PLCs) may be formed from a birefringent interferometric structure, such as the birefringent Mach-Zehnder PBS described in U.S. Pat. Nos. 7,035,491 and 7,356,206, both of which are incorporated herein by reference in their entirety. As depicted in FIG. 1, the basic configuration of an interferometric PBS includes an input coupler 114 having an input and two outputs and an output coupler 120 having two inputs and two outputs, and a pair of birefringent interferometric arms 116, 118 between the input and output couplers. The first arm 116 may couple the first output port of the input coupler 114 to the first input port of the output coupler 120, and the second arm 118 may couple the second output port of the input coupler 114 to the second input port of the output coupler 120. As depicted, the first arm 116 has a length L1 which differs from a length L2 of a second arm 118. The arms 116, 118 are “birefringent” with respect to each other such that the differential phase accumulation of two orthogonal polarization states propagating in the arms 116, 118 is correspondingly different. The differential phase accumulation, is typically the difference in phase between light having TE and TM polarizations propagating over length L1 in arm 116 and over length L2 in arm 118.
In operation, an optical signal may be provided as an input 112A to the input coupler 114. The coupler 114 splits the optical signal into a first optical signal which propagates on arm 116 and a second optical signal which propagates on arm 118, the first and second optical signals being of substantially the same optical power. In Mach-Zehnder interferometers, the amount of power exiting each of the first and second output ports of the output coupler 120, labeled “Output 1” and “Output 2” for example, may depend on the relative, or differential, phase between each of the two optical signals propagating in the arms 116, 118 at the corresponding input ports of the output coupler 120. In the special case that the differential phase is observed to be in multiples of 0°, for example 0°, 360°, etc., or mathematically 0°+N*360°, for any integer number N equal to or greater than 0, all the optical power will be directed out the first of the two output ports of the output coupler 120, and none out the second port. In the special case that the differential phase is observed to be in multiples of 180°, for example 180°, 540°, etc., or mathematically 180°+N*360°, for any integer number N equal to or greater than 0, all the optical power will be directed out the second port, and none out the first port.
The birefringent Mach-Zehnder PBS is typically designed such that for one polarization, the differential phase between first and second optical signals propagating through arm 116 and arm 118, respectively, is a multiple of 0°, while for the other orthogonal polarization the differential phase is a multiple of 180°. As a result, the optical signal associated with the first polarization will exit entirely at the first of the two output ports, Output 1 for example, of the output coupler 120, as an output signal 122A for example, while the optical signal associated with the other orthogonal polarization will exit entirely at the second of the two output ports, Output 2 for example, of the output coupler 120, as an output signal 122B for example.
Fabrication deviations and/or material ageing effects may impact the performance of the PBS over time, for example, resulting in the birefringent interferometric arms 116, 118 having differential phase lengths differing from the ideal lengths corresponding to relative phase multiples of 0° and 180°, as discussed above. With this degradation of the PBS performance, the two outputs of the output coupler 120 no longer include a single polarization state. Rather, a component of each polarization is present in the output signals 122A, 122B of the output coupler 120. Deviations away from the ideal design can be compensated by tuning one or both of the interferometric arms 116, 118, as discussed in commonly owned U.S. Pat. Nos. 7,035,491 and 7,356,206, both incorporated herein by reference in their entirety.
A thermo-optic effect is an often used tuning mechanism in optical devices. For instance, thermal energy in the form of heat may be applied to one of the interferometric arms 116, 118 to change the phase relationship of the propagating optical signals. This approach can be used to tune the phase of each of the optical signals propagating in the arms 116, 118 so that they are closer to the design criteria, e.g. multiples of 0° and 180° for the two polarizations, respectively, as described above. An electro-optic effect, for example where an electrical current or voltage changes the optical material index, may similarly be used for tuning Often, however, characteristics of the PBS may change over time, due to fabrication defects, device ageing or environmental conditions such as temperature or humidity for example, leading to performance degradation in the PBS.
What is needed is a PBS which can be tuned over a period of time commensurate with the use of the PBS to compensate for fabrication defects, device aging, or environmental operating conditions, which may lead to a corresponding performance degradation of the PBS.